Electrical Properties of Cobalt Monoxide

Abstract

The electrical conductivity of polycrystalline CoO1+x was measured as a function of temperature and partial pressure of oxygen, P(O2). The range of temperature was 900°—1450°C and the range of partial pressure was 1 − 10−12 atm. The plot of the conductivity against partial pressure of oxygen gave two distinct regions: Region A, where the conductivity is proportional to P(O2)¼, and Region B, where the conductivity is proportional to P(O2)⅙. From these results, together with considerations of ionic radii, we concluded that the dominant defects in CoO1+x at high temperatures are cation vacancies, with the majority being singly ionized in Region A and doubly ionized in Region B. A combination of gravimetric and electrical measurements showed that the mobility of the holes contributed by these vacancies rises exponentially with temperature. The activation energy of mobility is Eu=0.3 eV. At 1350°C, the mobility is about 0.4 cm2 V−1·sec−1. These values are discussed in terms of polaron theory. The sum of the enthalpy of formation of a neutral vacancy and the first ionization energy is Δhv+E1=0.56 eV. The second ionization energy was measured separately and found to be E2=0.65 eV. A rough estimate of the first ionization energy gives E1≅0.45 eV. Analysis of Seebeck-coefficient measurements performed in the same range of temperature and oxygen pressure is consistent with the results of the conductivity measurements regarding the sign and number of charge carriers. The term Ap, the so-called heat of transport, was calculated on the assumption that p0, the molar concentration of available states for charge carriers, is about 1. This assumption gave negative values for Ap. The term dAp/d(kT)−1, which is independent of the value of p0, is about 0.24 eV in Range A, and 0.54 eV in Range B. The value of dAp/d(kT)−1 in range A is close to Eμ, as the theory suggests. The resistivity of quenched samples of CoO1+x was measured at temperatures below 70°C for different x. Above the Néel temperature (tN≅19°C) the activation energy of the conductivity is ½E1+Eμ≅0.5 eV in a wide range of concentrations. This value is consistent with E1 and Eμ evaluated at high temperatures. At very low deviations from stoichiometry the activation energy increases, due perhaps to the appearance of electrons donated by interstitial cobalt atoms. Below the Néel temperature the activation energy ε decreases by about 0.1 eV, the difference between ε(t>tN) and ε(t<tN) depending on the departure from stoichiometry. This decrease is not understood. The appendix contains a slight correction to the phase diagram that was previously published. This correction is due to a more refined analysis of the data.

Point defect diagrams for pure and doped cobalt oxide $$ {\hbox{C}}{{\hbox{o}}_{{{1} - }}}_{\delta }{\hbox{O}} $$ in the temperature range of 1,173–1,673 K (I)

“…at which the value of the Seebeck effect should be zero ((empty square) points) [8,22,26,30]. As can be seen, at 1,373 and 1,473 K, a good consistency was obtained.…”

“…As can be seen in Fig. 1 were fitted in a way to obtain the consistency with the experimental data (considering the defects with a lower ionization degree) [5][6][7][8][9][10][11][12][13][14][15][16][17]. As can be seen in Fig.…”

“…From the studies on electrical conductivity and on the Seebeck effect, it transpires that the ratio of mobility of electrons vs. electron holes is about μ h /μ e =0.1 [4,8,11,21,22,26]. The oxygen pressure p…”

“…In order to determine this structure, the studies on the deviation from the stoichiometry were performed, using the electrochemical method [5,6] and the thermogravimetric method [7][8][9][10][11][12][13][14][15][16][17], as well as the studies on electrical conductivity [3,4,[7][8][9][10][11][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and on the Seebeck effect [4,9,11,16,22,30,33,34]. Similar characters of the relation of the deviation from the stoichiometry and the electrical conductivity on the oxygen pressure (the value of exponent near 1/4) were the basis for adopting the thesis that in the whole range of the oxide's existence the dominating defects are single-ionized cobalt vacancies and electron holes.…”

“…The present work treats the problem of the determination of the point defect diagram in non-stoichiometric cobalt oxide Co 1Àd O, pure and doped with M 3+ and M + metal ions, taking into consideration all of the types of defects in the cation sublattice, also interstitial cobalt ions, which have been postulated in several works. The bases for the diagrams' calculations were: a method prepared by the author of this work and the experimental values of the deviation from the stoichiometry in the temperature range of 1,173-1,673 K, obtained by many authors [5][6][7][8][9][10][11][12][13][14][15][16][17].…”